The present invention relates to a magnetic resonance imaging apparatus and a magnetic resonance imaging method, and particularly to a magnetic resonance imaging method and a magnetic resonance imaging method each of which executes an actual scan for allowing an RF coil unit to transmit an RF pulse to an imaging area of a subject in a static magnetic field space and to receive a magnetic resonance signal generated in the imaging area to which the RF pulse is transmitted, and thereafter generates an actual scan image about the imaging area, based on the magnetic resonance signal obtained by the execution of the actual scan.
A magnetic resonance imaging (MRI: Magnetic Resonance Imaging) apparatus has frequently been used particularly in medical applications as an apparatus which photographs an image about a tomographic plane of a subject through the use of a nuclear magnetic resonance (NMR: Nuclear Magnetic Resonance) phenomenon.
In the magnetic resonance imaging apparatus, a subject is accommodated in a static magnetic field space to align spins of proton of the subject in a static magnetic field direction, thereby generating magnetization vectors. Then, an RF pulse having a resonant frequency is applied to generate a nuclear magnetic resonance phenomenon, thereby changing the magnetization vectors of the proton. Thereafter, the magnetic resonance imaging apparatus receives a magnetic resonance (MR) signal generated when the proton is returned to its original magnetization vector state, and generates a tomographic image about a tomographic plane of the subject, based on the received magnetic resonance signal.
As an RF receiving coil for receiving the magnetic resonance signal in the magnetic resonance imaging apparatus, a surface coil such as a phased array coil or the like is frequently used. However, the surface coil has such a characteristic that receiving sensitivity is reduced with distance from a source of generation of the magnetic resonance signal in the subject. A sensitivity distribution in the entire imaging area is not uniform spatially. Therefore, there is a case in which artifacts occur in a tomographic image generated using the magnetic resonance signal received by the surface coil, thereby degrading the quality thereof.
Therefore, the tomographic image is correction-processed using a reception sensitivity distribution in order to cope with a problem or trouble caused by reception-sensitivity non-uniformity of such a surface coil. Described specifically, a reference image is acquired by executing a reference scan in addition to an actual scan, and a reception sensitivity distribution in an imaging area of the surface coil is measured using the reference image. Thereafter, a tomographic image generated by the actual scan is corrected using the measured reception sensitivity distribution (refer to, for example, a patent document 1).
There is however, for example, a case in which since a high frequency magnetic field formed by transmitting an RF pulse by means of an RF transmitting coil such as a body coil might be ununiform due to a dielectric constant effect upon imaging a subject in a high static magnetic field space having a magnetic field strength of 3 Tesla or higher, the removal of artifacts cannot be attained adequately even when the tomographic image is corrected using the above reception sensitivity distribution. That is, there is a case in which it is difficult to improve image quality due to the fact that a transmission sensitivity distribution is spatially ununiform.
Thus, a transmission sensitivity distribution is measured and the tomographic image is corrected using the measured transmission sensitivity distribution. The transmission sensitivity distribution is measured by, for example, a Double flip angle method. Described specifically, a plurality of reference scans are executed at flip angles different from one another. A transmission sensitivity distribution is measured using reference images obtained by the respective reference scans. Thereafter, a tomographic image acquired by an actual scan is correction-processed based on the transmission sensitivity distribution, thereby suppressing the occurrence of artifacts (refer to, for example, a non-patent document 1 and a non-patent document 2).
[Patent Document 1] Japanese Unexamined Patent Publication No. 2005-177240
[Non-Patent Document 1] Hiroaki Mihara et. al., A method of RF inhomogeneity correction in MR imaging, Magnetic Resonance Materials in Physics, Biology and Medicine 7, USA., 1998, p. 115-120
[Non-Patent Document 2] Jinghua Wang et. al., In vivo method for correcting transmit/receive nonuniformities with phased array coils, Magnetic Resonance in Medicine 53, USA., 2005, p. 666-674
Using the reception sensitivity distribution and the transmission sensitivity distribution in this way, the tomographic image obtained by the actual scan is corrected to enhance the quality thereof.
However, the transmission sensitivity distribution might cause an error due to influences such as susceptibility artifacts and the difference in T1 value between tissues. In particular, the transmission sensitivity distribution depends upon explicit flip angles. Since the gain increases with a decrease in flip angle, the error becomes larger. That is, the transmission sensitivity distribution measured in the above-described manner contains a portion low in accuracy. Therefore, when correction processing is made using such a transmission sensitivity distribution, an artifact might additionally occur in an image. Thus, there was a case in which an improvement in image quality was difficult.